A precipitation hardened high Ni heat-resistant alloy including Ni matrix having finely precipitated therein an intermetallic compound is known as an alloy material of heat-resistant parts requiring high temperature mechanical strength, such as engine valves of automobiles and turbine wheels for thermal power generation. The high Ni heat-resistant alloy contains, in a component composition thereof, precipitate-forming elements such as Al, Ti and Nb that form an intermetallic compound with Ni, but those elements have strong bonding force to C and are easy to form carbide. For this reason, in the high Ni heat-resistant alloy containing C, excellent high temperature mechanical strength by the intermetallic compounds is obtained, but on the other hand, deterioration of cutting workability (machinability) by the carbide precipitated becomes a problem.
To improve machinability of the high Ni heat-resistant alloy, alloy component composition including carbon and precipitation-forming elements is severely controlled and adjusted using a vacuum melting furnace, a remelting furnace or the like. Furthermore, it is proposed to control an aging heat treatment that controls precipitation state of precipitates.
For example, Patent Document 1 discloses a precipitation hardened Ni-based heat-resistant alloy for hot work die containing Al, Ti and Nb, which is high Ni heat-resistant alloy containing Cr in an amount of about 14 to 25 mass %, in which C content is controlled to 0.03 mass % or less to suppress precipitation of carbide, whereby machinability can be improved. This patent document describes about poor machinability of an alloy having a component composition precipitating intermetallic compounds called “γ′ phase” and “γ″ phase” in addition to γ phase, among high Ni heat-resistant alloys. The patent document describes that the poor machinability is generally considered to be due to hardness of fine particles including intermetallic compounds as a hardening phase, but the influence by primary carbide precipitated in a solidification process during casting an alloy is great, and also describes that the amount of C is controlled in order to suppress precipitation of the primary carbide.
Similar to Patent Document 1, Patent Document 2 discloses a method for improving machinability of high Ni heat-resistant alloy containing Cr in an amount of about 14 to 25 mass % by dissolving precipitated fine primary carbide in a matrix by a soaking heat treatment. The patent document describes that the soaking heat treatment is, for example, a heat treatment of maintaining an alloy in a relatively high temperature region as compared with a melting point of the alloy, such as 1,100 to 1,300° C., for a long period time of 10 to 40 hours, and by this treatment, Michelin point as an index of cleanliness of an alloy can be reduced to 100 or less.
Patent Document 1: JP-A-2009-167499
Patent Document 2: JP-A-2009-167500